There has been an increasing interest in the development of laser sources that emit in the mid-infrared (“mid-IR”) spectral region, particularly at wavelengths between about 2.5 and 6 μm. Such lasers have significant uses for both military and non-military applications. In the military realm, mid-IR lasers can be extremely useful as a countermeasure to jam heat-seeking missiles and prevent them from reaching their targets. In both the military and non-military realm, such mid-IR lasers have found use, for example, in chemical sensing, and so may be very useful in environmental, medical, and national security applications.
On the short-wavelength side of this spectral region, type-I quantum-well antimonide lasers are achieving excellent performance and greater maturity. See, e.g., T. Hosoda, G. Kipshidze, L. Shterengas and G. Belenky, “Diode lasers emitting near 3.44 μm in continuous-wave regime at 300K,” Electron. Lett. 46, 1455 (2010). On the long-wavelength side of the mid-IR, intersubband quantum cascade lasers (QCLs) have become the dominant source of laser emissions. See, e.g., Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Room-temperature continuous wave operation of distributed feedback quantum cascade lasers with watt-level power output, Appl. Phys. Lett. 97, 231119 (2010).
In recent years, the interband cascade laser (ICL) has been developed as another promising semiconductor coherent source in the mid-IR range.
The first ICLs were developed by Dr. Rui Yang in 1994. See U.S. Pat. No. 5,588,015 to Yang. The ICL may be viewed as a hybrid structure which resembles a conventional diode laser in that photons are generated via the radiative recombination of an electron and a hole. However, it also resembles a quantum cascade laser in that multiple stages are stacked as a staircase such that a single injected electron can produce an additional photon at each step of the staircase. See S. Slivken, Y. Bai, S. B. Darvish, and M. Razeghi, “Powerful QCLs eye remote sensing,” Compound Semiconductor, pp. 22-23(2008); see also U.S. Pat. No. 5,457,709 to Capasso et al.
Each stage of an ICL is made up of an active quantum well region, a hole injector region, and an electron injector region. The photon cascade is accomplished by applying a sufficient voltage to lower each successive stage of the cascade by at least one quantum of photon energy ℏω and allowing the electron to flow via an injector region into the next stage after it emits a photon having that energy. See J. R. Meyer, I. Vurgaftman, R. Q. Yang and L. R. Ram-Mohan, “Type-II and type-I interband cascade lasers,” Electronics Letters, Vol. 32, No. 1 (1996), pp. 45-46 (“Meyer 1996”); and U.S. Pat. No. 5,799,026 to Meyer et al., both of which are incorporated by reference into the present disclosure.
ICLs also employ interband active transitions just as conventional semiconductor lasers do. Each interband active transition requires that electrons occupying states in the valence band following the photon emission be reinjected into the conduction band at a boundary with semi-metallic or near-semi-metallic overlap between the conduction and valence bands. Most ICLs to date employ type-II active transitions where the electron and hole wavefunctions peak in adjacent electron (typically InAs) and hole (typically Ga(In)Sb) quantum wells, respectively, though ICLs employing type-I transitions where the electron and hole wavefunctions peak in the same quantum well layer have also been developed. See U.S. Pat. No. 5,799,026 to Meyer et al., supra.
In order to increase the wavefunction overlap in type-II ICLs, two InAs electron wells often are placed on both sides of the Ga(In)Sb hole well, and create a so-called “W” structure. In addition, barriers (typically Al(In)Sb) having large conduction- and valence-band offsets can surround the “W” structure in order to provide good confinement of both carrier types. See U.S. Pat. No. 5,793,787 to Meyer et al., which shares an inventor in common with the present invention and which is incorporated by reference into the present disclosure in its entirety. Further improvements to the basic ICL structure include using more than one hole well to form a hole injector. See U.S. Pat. No. 5,799,026 to Meyer et al., supra.
Additional early improvements to ICL design include those described in R. Q. Yang, J. D. Bruno, J. L. Bradshaw, J. T. Pham and D. E. Wortman, “High-power interband cascade lasers with quantum efficiency >450%,” Electron. Lett. 35, 1254 (1999); R. Q. Yang, J L. Bradshaw, J. D. Bruno, J. T. Pham, and D. E. Wortman, “Mid-Infrared Type-II Interband Cascade Lasers,” IEEE J. Quant. Electron. 38, 559 (2002); and in K. Mansour, Y. Qiu, C. J. Hill, A. Soibel and R. Q. Yang, “Mid-infrared interband cascade lasers at thermoelectric cooler temperatures,” Electron. Lett. 42, 1034 (2006).
However, the performance of the first ICLs fell far short of the theoretical expectations. In particular, the threshold current densities at elevated temperatures were quite high (5-10 kA/cm2 at room temperature in pulsed mode) and fell only gradually to 1-2 kA/cm2 for a relatively large number of stages, which precluded room-temperature continuous-wave (cw) operation of those devices.
More recently, researchers at the U.S. Naval Research Laboratory (NRL) formulated and tested certain design changes tuning the configuration of the hole injector region within a given stage, the active quantum wells within a given stage, the electron injector region within a given stage, the active gain region comprising multiple stages, and/or the separate confinement region. These design changes have dramatically improved ICL performance, with the threshold current density falling to approximately 400 A/cm2 and the threshold power density to approximately 900-1000 W/cm2. See U.S. Pat. No. 8,125,706 Vurgaftman et al.; U.S. patent application Ser. No. 13/023,656 Vurgaftman et al. filed Feb. 9, 2011; and U.S. patent application Ser. No. 13/353,770 Vurgaftman et al. filed Jan. 19, 2012, all of which share at least one inventor in common with the present invention and are hereby incorporated by reference into the present disclosure in their entirety. As a consequence, in early 2010, an NRL device reached a maximum cw operating temperature of 72° C., which was 60° C. higher than for any ICL designed elsewhere.
NRL's research on further improvements to ICL performance has continued.